This is a lovely park located in the north of the city. To access it: walk up the Soanish Step, turn left at the too and it is a couple of minutes walk along the road up on the right. From the gardens there is lovely views over Rome and especially Piazza Popolo. The park extends for miles so you can walk for hours around it (it's the third biggest park in Rome).
There are often buskers playing which really adds to the atmosphere. The last time I was here there was a young girl playing a piano, who was amazing for such a young age. It was a beautiful day so wandering around the park with a piano playing in the background was just idilic.